The present invention is directed generally to an optical fiber probe and methods measuring optical properties. In particular, the present invention is directed to an optical fiber probe and methods for observing light diffusely reflected from an object at spatially separated locations.
Optical methods have been widely used to characterize the physical and chemical properties of materials. These methods are typically based on various modes of interaction of light with a sample. These interactions can be described as various optical properties of the sample and can often be used to identify or distinguish different materials.
Optical methods can be used in industrial environments to monitor the progress of chemical reactions and the concentration of the various components in a process stream. Additionally, optical methods can be used to determine trace impurities in chemicals. In many instances, samples must be removed from a reactor or process stream for analysis. New non-destructive and non-invasive characterization methods are needed to monitor reactions on a continuous basis.
Many conventional optical techniques are based on the absorption of light by a chemical species. The amount of light absorbed is often proportional to the concentration of the chemical present. Typically, absorption of light is determined by measuring the amount of light transmitted through a sample. A disadvantage of many techniques based on determining the absorption or transmission of light is the need for dilute samples to obtain accurate quantitative results. If the concentration is too high, scattering becomes significant and/or transmission of light through the sample is low. Consequently, industrial chemical process streams frequently are not amenable to absorption techniques because of high concentrations or turbidity.
Another optical property that can be observed is fluorescence in which a chemical species absorbs light of a particular wavelength and emits light at another wavelength. Unlike absorption techniques that generally measure the amount of transmitted light, fluorescence techniques do not require that the illumination light beam pass through the sample. This permits the application of fluorescence techniques to turbid or opaque samples.
Conventional fluorescent techniques have been used in medical diagnosis. In some instances, cells or tissue have natural chromophores so that dyes may not be needed to detect abnormal cells or tissue. However, in many cases, the tissue or cells must be brought in contact with a dye that is selectively retained by either diseased or healthy cells or tissue. The dye fluoresces when irradiated and helps locate abnormal tissue. To be effective, a dye must be found that is selectively retained by the diseased or healthy cells or tissue and is also suitable for use in humans.
Although, illumination by light at multiple wavelengths can provide a characteristic fluorescence spectrum of the tissue or cells, in many cases, the amount of fluorescent light reaching a detector is very low because of low quantum yields for fluorescence. Additionally, because fluorescent light is emitted uniformly in all directions, only a small portion of the light is typically directed toward a detector. Therefore, there is a need to provide new probes and techniques for the determination of optical properties of samples, including, for example, the diagnosis of cells or tissue and/or the characterization of components in an industrial chemical process stream.
Generally, the present invention relates to a method of determining optical properties, including scattering and/or absorption properties, to characterize samples, including medical and industrial samples, and a probe for use in the method. One embodiment is a probe including an illumination fiber to radiate light toward an object and at least two collection fibers to receive light from the object. At least two of the collection fibers are spaced at different distances from the illumination fiber.
Another embodiment of the invention is a probe assembly including a light source, an illumination fiber coupled to the light source to radiate light toward an object, two or more collection fibers to receive light from the object, and a detector for measuring a characteristic, such as the intensity, of the light from the collection fiber. At least two of the collection fibers are spaced at different distances from the illumination fiber.
Yet another embodiment is a method of characterizing a region of a sample. The region is illuminated with light from an illumination fiber. A portion of the light is diffusely reflected by the region. A portion of the diffusely reflected light is received by two or more collection fibers, at least two of which are spaced at different distances from the illumination fiber. The region is characterized based on an amount of light received by each of the collection fibers. When the method is used for medical diagnostic purposes, the characterization of the region may be used to make a diagnosis.